eISBNs and ebook formats

In Does Your Ebook Need Its Own ISBN? (BookBuzzr), the suggestion is made that a publisher should assign a different ISBN (eISBN) to each different electronic format that an ebook is published in.

Since EPUB format (can be read on a NOOK) and MOBI format (can be read on a Kindle) are interconvertible with Calibre for example (see Calibre manual manual.calibre-ebook.com), does it make sense that you have to have one eISBN for the EPUB version and another eISBN for the MOBI version of the "same" ebook? It makes sense — I guess — that there is a different ISBN for a hardback and a paperback version of a book, and each of these from an ebook version, but if the conversion between two ebook formats is just a software conversion process, does it make sense that an ebook in the two formats needs a different ISBN for each format?

I would think that since it's only a software conversion between the two formats (EPUB and MOBI in this case), that you would not need to use two ISBNs. After all, a consumer with the EPUB version can easily make a MOBI version to read on a Kindle — assuming there is no DRM lock on the EPUB version, of course.

(Of course when MOBI goes away and Kindle is EPUB, then this will no longer be an issue.) :)